It is known that a very high-contrast photographic image can be formed using a certain kind of silver halide, and various methods for forming such a photographic image are employed in the field of photo mechanical processes.
According to one method, a lith-type silver halide light-sensitive material comprising silver chlorobromide (at least 50 mol % of which is silver chloride) is processed with a hydroquinone developer having a very low effective concentration of sulfite ions (usually not more than 0.1 mol/liter) so as to obtain a halftone image or line original having high contrast and optical density wherein image areas are clearly distinguished from nonimage areas. However, the development achieved by this method is extremely liable to aerial oxidation because of the low sulfite concentration in the developer, and various efforts and approaches have been necessitated to stabilize the activity of the developer.
Under the situation described above, it has been desired to devise an image-forming system that eliminates the instability in image formation by the aforementioned developing method (lith development system) and which provides super high contrast photographic characteristics by development with a processing solution having improved stability during storage. In response to this need, systems capable of forming super high contrast negative images (.gamma.&gt;10) have been described in U.S. Pat. Nos. 4,166,742, 4,168,977, 4,221,857, 4,224,401, 4,243,739, 4,272,606, and 4,311,781; according to the described methods, surface latent-image type silver halide photographic materials to which specified acylhydrazine compounds have been added are processed at a pH between 11.0 and 12.3 with developers that contain at least 0.15 mol/liter of a preservative sulfurous acid and which exhibit good stability during storage. The new image-forming systems permit the use of silver iodobromide or silver chloroiodobromide, which is a definite advantage over the previously described methods for forming super high contrast images which can be implemented solely by the use of silver chlorobromide with high silver chloride content.
With the commonly employed silver halide photographic materials, the optical density of an image obtainable per unit amount of developed silver increases in inverse proportion to the size of silver halide grains but, on the other hand, the sensitivity of a silver halide generally increases in the size of silver halide grains. Therefore, in order to obtain a light-sensitive material that provides both high sensitivity and high optical density, it is necessary that a silver halide emulsion having a large grain size be incorporated in an increased amount per unit area. However, light-sensitive material containing a larger amount of silver halide emulsion is not adaptive to rapid processing, since it takes a prolonged time to fix, wash, and dry the developed material. In addition, silver is a precious metal whose world production and reserves are limited, so that it is economically advisable to produce light-sensitive materials with minimum use of silver.
Under these circumstances, studies have been conducted for many years in order to realize the production of silver halide light-sensitive materials having high image density and sensitivity while using silver in a reduced amount.
The new image-forming systems described above which are capable of forming extremely high contrast images in high speed have a serious disadvantage in that they sometimes cause the formation of "black pepper" due to infectious development, and this presents a problem in photomechanical processes.
"Black pepper" refers to the formation of tiny black spots of developed silver that occur in the areas which would otherwise be unexposed nonimage areas. Black pepper tends to occur more frequently if the pH of the processing solution increases, for reasons such as fatigue with time. While extensive efforts have been made in order to prevent the occurrence of black pepper, success in reducing the formation of black pepper is often accompanied by a decrease in sensitivity or image contrast. Therefore, it has been desired to devise a system that maintains high-sensitivity and super high contrast photographic characteristics but yet is capable of satisfactory prevention of black pepper.